This invention relates to a method of, and apparatus for, measuring the velocity of a fluid.
A common technique for measuring the velocity of a fluid flowing in a fluid path is to provide two ultrasonic transducers on opposite sides of the fluid path, one of the transducers being upstream relative to the other, and measuring the fluid velocity according to the difference in transit time of an ultrasonic signal in the upstream and downstream directions. This technique is capable of providing an accurate measure of the velocity of a fluid, and a relatively large number of systems have been designed to implement this technique, including those disclosed in U.S. Pat. Nos. 2,991,640, 3,097,526, 3,402,606, 3,440,876, 3,473,378, 3,631,719, 3,678,731, 3,906,791, 3,918,304, 3,935,735, 4,052,896, 4,312,239, 4,334,434, 4,367,654, 4,446,744, 4,475,406, 4,483,202, 4,545,244, 4,860,593 and 5,325,726
However, many of these prior art fluid velocity measuring systems employed elaborate signal processing to measure and compare transit times. Others of these systems had to be calibrated for a specific fluid, under specific conditions, and thus could not reliably measure fluid under changing conditions.
The present invention relates to an improved method of, and apparatus for, measuring the velocity of fluid flowing in a fluid path. Generally the method of the present invention comprises the steps of simultaneously applying equal but opposite input signals to two ultrasonic transducers acoustically coupled diagonally across the fluid path to cause the transducers to generate ultrasonic signals and combining the output signal of each transducer, caused by the ultrasonic signal transmitted by the other transducer across the fluid path, to obtain a destructive interference signal and a constructive interference. Then the amplitude of the destructive interference signal is compared with the amplitude of the transducer output signal (i.e., the output signal of either of the transducers) to obtain a measure of the velocity of the fluid.
Generally, the apparatus for measuring fluid velocity according to the present invention comprises a signal generator for generating input signals, and two ultrasonic transducers acoustically coupled diagonally across the fluid path, and connected in series to the signal generator such that equal and opposite input signals are simultaneously applied to the transducers. The apparatus also includes a signal processor for combining the output signals of each transducer caused by the ultrasonic signal transmitted by the other transducer across the fluid path to obtain a destructive interference signal and a constructive interference signal. The signal processor also compares the amplitude of the destructive interference signal with the amplitude of the transducer output signal to obtain a measure of the velocity of the fluid.
By simultaneously providing the transducers with equal but opposite input signals, and combining the output signals to obtain destructive and constructive interference signals, the method and apparatus of the present invention greatly simplify the processing of signals, and the determination of the fluid velocity. The method and apparatus of the present invention allow the received signals to be demodulated without reference to the phase of the excitation signal used to create the input signals. Furthermore, with the method and apparatus of the present invention there is no need for calibration against an independent reference signal. The difference in transit time can be calculated directly from the amplitude and frequency of the output signals from the transducers.
The method of, and apparatus of, this invention is particularly adapted for measuring fluid flow, such as blood through a conduit or vessel.
These and other features and advantages will be in part apparent, and in part pointed out, hereinafter.